"Package 1 has failed validation ... 1021001 No rating rules found for provided combination. Country Not Found." message when mailing to Puerto Rico in SendPro Enterprise

The message "Package 1 has failed validation. Error message from PB Shipping API: 1021001 No rating rules found for provided combination. Country Not Found." occurs when mailing to Puerto Rico because First Class Mail International was selected instead of First Class Mail.
Products affected: SendPro® Enterprise

Issue

When printing a stamp to Puerto Rico from the Stamps app, after selecting the rate button, the message "Package 1 has failed validation. Error message from PB Shipping API: 1021001 No rating rules found for provided combination. Country Not Found." appears.

Cause

First Class Mail International was selected instead of First Class Mail.

Resolution

Because Puerto Rico is a US Territory, you need to select First Class Mail from the Service menu, not First Class Mail International.

UPDATED: June 19, 2023